Thorium- and Uranium-Mediated C–H Activation of a Silyl-Substituted Cyclobutadienyl Ligand

Cyclobutadienyl complexes of the f-elements are a relatively new yet poorly understood class of sandwich and half-sandwich organometallic compounds. We now describe cyclobutadienyl transfer reactions of the magnesium reagent [(η4-Cb'''')Mg(THF)3] (1), where Cb'''' is tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl4 proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η4-Cb'''')AnCl(μ-Cl)3Mg(THF)3] (An = Th, 2; An = U, 3). Using a 2:1 reaction stoichiometry produces [Mg2Cl3(THF)6][(η4-Cb'''')An(η3-C4H(SiMe3)3-κ-(CH2SiMe2)(Cl)] (An = Th, [Mg2Cl3(THF)6][4]; An = U [Mg2Cl3(THF)6][5]), in which one Cb'''' ligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An–C σ-bond, protonation of the four-membered ring, and an η3-allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of 4 and 5 indicates that the cyclobutadienyl ligands undergo C–H activation across the actinide center.

with a venting needle and left in the glovebox at room temperature to slowly evaporate. After two days, large orange crystals of 2 were formed, which were then isolated by removing the supernatant with a pipette, washed with n-heptane, dried in a stream of argon in the glovebox, followed by drying in vacuo for 15 minutes. If an oil formed during crystallisation or the solution became cloudy, dropwise addition of THF until a bright orange solution was obtained, followed by repeating the slow evaporation, allowed crystals of 2 to form (89 mg, 65%).

X-ray Crystallography
Data on 2 were collected on a Rigaku FR-007HF rotating anode equipped with a Saturn 724+ CCD area detector and a quarter-chi goniometer performing  scans to fill the Ewald sphere at 100 K, using Cu/K radiation. The power of the anode was reduced to 0.7 kW, since at its nominal operating power (1.2 KW) the crystals suffered from extended radiation damage. Data collections for 3, [Mg 2 Cl 3 (THF) 6 ] [4] and [Mg 2 Cl 3 (THF) 6 ] [5] were carried out on a Rigaku Gemini Ultra diffractometer equipped with an EOS-CCD detector and a four-circle kappa goniometer performing  scans to fill the Ewald sphere at 100 K, using Cu/K radiation. Crystals were mounted on MiTiGen loops from pump oil kept over activated 4 Å molecular sieves in a glove box. Data collection and processing was handled by CrysAlis Pro. Data for 2 and [Mg 2 Cl 3 (THF) 6 ] [4] were collected to 0.83 Å resolution, while for 3 and [Mg 2 Cl 3 (THF) 6 ] [5] to 0.82 Å resolution. In the case of 3, [Mg 2 Cl 3 (THF) 6 ] [4] and [Mg 2 Cl 3 (THF) 6 ] [5] an analytical absorption correction based on crystal faces (Clark and Reid 4 ) was applied. Structure solution and model refinement were performed using the Olex2 package 5 and all software within.
In the case of [Mg 2 Cl 3 (THF) 6 ] [4], the model shows significant disorder in both the anionic and cationic part. Specifically, in the [Mg 2 (-Cl) 3 (THF) 6 ] + counter-cation all the chlorides and most of the coordinated THF molecules show disorder between two positions. These disorders were modelled using the PART command and treated with SADI restraints. In the case of the THF molecules, some carbon and oxygen atoms had to be refined isotropically using the ISOR command for the refinement to converge and be stable. Furthermore, the anisotropic displacement parameter of atom C51a had to be constrained to the ADP of its PART 1 partner C51 using the EADP command. The anionic part 4 also features significant disorder with the  4 -Cb'''' ligand, which was modelled as occupying two positions. In order to achieve a stable and converging refinement a RIGU restraint had to be used. All other disorder was treated using SADI and, where necessary, ISOR restraints. An electron density peak of 3.1 e Å -3 1.603 Å away from the thorium atom was observed; this has no chemical meaning and is attributed to absorption effects. The protons of the cyclometallated CH 2 group were found in the electron density map and refined freely.
In the case of [Mg 2 Cl 3 (THF) 6 ] [5], the dataset was treated as a two component non-merohedral twin (180˚ rotation along the 0 1 0 direct space axis) with a 90:10 component fraction. Only the major component of the twin was used to generate the hkl4 file, which was then used for the initial solution and refinement. For the final refinement the hkl5 file was used (see Table S1). A common scaling factor was used for both components. Furthermore, a highly disordered half-molecule of THF occupying a special position was identified in the electron density difference map, which could not be appropriately modelled and was excluded using the solvent mask utility in Olex2. Minimal disorder was observed in two of the coordinated THF molecules in the [Mg 2 (μ-Cl) 3 (THF) 6 ] + cation which was treated using SADI and, where appropriate, ISOR restraints. The final model exhibits an electron density peak of 4.4 e Å -3 1.02 Å from the uranium atom, which has no chemical meaning and is attributed to absorption from the uranium atom and poor separation of the reflections stemming from the two components especially at low resolutions. The cyclometallated carbon C1 was refined isotropically. A model where the dataset was not treated as a twin gave better final R 1 and wR 2 (5.01 and 13.05), but it displayed a somewhat high b parameter for the weighting scheme (35.9). The final model displayed also an electron density peak of 2.5 e Å -3 2.724 Å from uranium and 0.88 Å away from the cyclometallated carbon C1 (which has an oblate ellipsoid). Treatment of the dataset as the two-component twin described above resulted in the latter electron density peak disappearing. Furthermore, the b factor of the weighting scheme was almost zero.

Computational details
Optimizations of thorium and uranium complexes were carried out by employing the DFT hybrid functional (B3PW91) 6 along with the small core pseudopotential Stuttgart basis set for uranium, thorium, chlorine, and silicon atoms with additional polarization functions for chlorine and silicon atoms. 7 Pople basis sets (6-311+G** basis set for magnesium and 6-31G** for carbon, nitrogen, oxygen, hydrogen atoms) were employed for other atoms. 8 Frequency calculations were performed to locate minima (maxima for transition states) for the optimized structures. All the calculations were performed using Gaussian 09 suite of programs. 9